Several drugs that are used clinically in the treatment of cardiovascular disease act at the vascular alpha1-adrenoceptors. It is important, especially from the stand-point of drug design, to classify and characterize these receptors. This proposal is directed toward this goal and will lead to a greater understanding of the molecular events involved in drug action. On the basis of current pharmacological evidence, it is proposed that vascular alpha1-adrenoceptors exhibit species, tissue and blood vessel heterogeneity. Since the classification of receptors will ultimately involve a knowledge of their pharmacological properties and receptor structure the hypothesis will be explored using these two criteria. The affinities of a series of alpha-adrenergic ligands for alpha1- adrenoceptors will be determined in rat and rabbit arteries in radioligand binding experiments. Evidence for alpha1- adrenoceptor heterogeneity will come from differences in ligand affinities with regard to rank order of potency and/or absolute values. Since variations in the receptors' microenvironment are known to affect the binding and intrinsic activities of both agonists and antagonists, the affinities of the ligands will be determined in broken cell membrane and detergent solubilized receptor preparations as well as in the presence of known receptor modulators. The hypothesis will be supported if it is found that ligands retain their discriminating actions under these conditions. Secondly, it may be possible to distinguish between subpopulations of alpha1-adrenoceptors on the basis of certain gross molecular parameters such as their molecular size, isoelectric points, hydrodynamic properties and glycoprotein nature. This proposal is directed toward the long-term objectives of classifying and fully characterizing vascular receptors and will form the basis for the arduous task of receptor purification.